The present invention relates to a control system for an electromagnetic clutch for an automatic transmission of a motor vehicle.
An automobile provided with a continuously variable belt-drive transmission with an electromagnetic clutch is disclosed in EP-A No. 151038. The electromagnetic clutch of the transmission is controlled by a control system to provide various operational modes for clutch torque (clutch current). One of the modes is selected in accordance with a position of a selector lever and driving conditions of the vehicle to control the electromagnetic clutch.
The operational modes include a start mode and a drag mode decided in accordance with operations of an accelerator pedal at a vehicle speed lower than a predetermined speed. In the start mode of the vehicle, the clutch current is increased in proportion to the engine speed or the opening degree of a throttle valve of the engine for smoothly engaging the clutch. In the drag mode, the clutch current is reduced to zero or a small current to disengage the clutch or to provide a drag torque.
More particularly, when the accelerator pedal is released in a low vehicle speed range after starting the vehicle to close a throttle valve (FIG. 7a), the mode of the clutch current is immediately changed to the drag mode. Thus, as shown in FIG. 7b, the clutch current becomes zero or is changed to a drag current id to disengage the clutch. The engine speed Ne gradually decreases, while speed No of an output shaft of the clutch begins to oscillate as shown in FIG. 7c. The reason for the oscillation is as follows.
At a start of the vehicle, driving torque is applied to the drive power transmission system of the vehicle. When the accelerator pedal is released, the driving torque is exerted on the system in the reverse direction. At this time, the clutch is disengaged, so that the system is cut off, which causes the oscillation of the system. The oscillation causes the vehicle to stumble, which is transmitted to the driver Therefore, the driveability at coasting in a low vehicle speed range is not good.
Particularly, in an automobile with a continuously variable belt-drive transmission, since a pulley device has a large mass of inertia, such a stumbling may easily occur.